1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to an electroluminescence (EL) device made from a graded layering of silicon (Si) nanocrystal (nc) embedded Si oxide (SiOx) films.
2. Description of the Related Art
FIG. 1 is a schematic block diagram of an electroluminescence device made from a Si rich SiOx emissive layer (prior art). The current state of the art in silicon photonics uses silicon nanocrystals embedded in a silicon dioxide (SiO2) matrix. Such a material is fabricated by several methods. One simple fabrication method is by the ion implantation of Si ions into SiO2 films, and another is by plasma-enhanced chemical vapor deposition (PECVD) of excess Si into oxide films. The EL device 100 has an active emissive layer 102 that is deposited onto a conductive substrate 104, such as a doped Si wafer that serves as a bottom electrode.
This emissive layer 102 is a film in which an excess of Si is deposited in concert with SiO2, and is subsequently annealed. The excess Si aggregates into distinct phases of Si nanocrystals on the order of 5 nanometers (nm) to 10 (nm). These nanophases of Si provide a quantum confinement that permits the radiative decay of excited optical states. After the deposition of the active emissive layer 102, a transparent conductive electrode 106, such as indium tin oxide (ITO), is deposited, which permits the transmission of the device optical output under bias.
With this construction, charge carriers (electrons and holes) are injected from both electrodes 104/106 and form excited optical states in the Si nanocrystals (nc-Si) called excitons. If the nc-Si phases are of the proper size and distribution, the excitons decay into optical emission or electroluminescence. Conventionally, a single emissive layer is formed (as shown), which is exclusively responsible for carrier transport, exciton formation, and subsequent radiative decay. Based on empirical data, it has been determined that the balancing all of these interactions is difficult within a single homogeneous film. A nc-Si in-oxide material that is efficient in transporting charge carriers is not necessarily efficient in allowing excitons to decay into light emission. Emissive films with a higher concentration of nc-Si particles (e.g., greater than 20% excess) are better conductors than light emitters, since the close proximity permits many of the nano-particles in the excited state to simply dissipate into heat or current. On the other hand, a lower nc-Si concentration (e.g., 5-10% excess) does permit exciton radiative decay if inter-particle spacing is sufficient. Generating sufficient current to form excitons on nc-Si particles is facilitated using a high nc-Si in-oxide concentration material, but this same material will not support the radiative decay of excitons with the same efficiency as a lower concentration nc-Si Si oxide material.
It would be advantageous if an EL device could be made from a Si rich Si oxide emissive layer that supports both radiative decay, as well as the generation of excitons.